You probably think you already know everything about The Avengers. After all, the Marvel superhero team-up movie had years of anticipation and build-up — which actually paid off, with a movie you probably saw more than once. But there are still secrets you'd never have guessed about Joss Whedon's massive aliens-vs-heroes spectacle.

With The Avengers coming to DVD and Blu-ray on Sept. 25, we sat down with three of the movie's VFX creators at Industrial Light and Magic, and learned some totally insane secrets of the film. We also got a sneak peek at the creation of some CG sequences, like the above behind-the-scenes video about the making of the Hulk.

Here are 24 strange, thrilling secrets about The Avengers.

Last month, we journeyed to ILM's headquarters at the Lucasfilm campus, and watched some behind-the-scenes videos from The Avengers (which we're now able to share here.) We spoke to Visual Effects Supervisor Jeff White, Associate Visual Effects Supervisor Jason Smith, and Animation Director Marc Chu. (We spoke to all of them in both group interviews and exclusive one-on-one interviews.)

Here's everything they told us about the Hulk, Iron Man, and the streets of New York:

The Hulk

1. The Hulk started out a bodybuilder wearing green body paint.
Well, sort of. "We actually had a muscley guy on set, shirtless and painted green," says Jeff White. This was so that they could see how the green skin worked with the actual physical sets, and look realistic in context. "The guy they had on set really got into being referenced. He was flexing constantly as he went around," says White — and it sounds like he was pretty popular with many of the crew.

2. There was also a professional athlete in Mark Ruffalo's mo-cap suit.
It wasn't just Mark Ruffalo in the motion-capture suit for the Hulk. At various points, "everybody" wore the Hulk's mo-cap suit, says Marc Chu — even Chu himself wore it. For the scene where Bruce turns into the Hulk and chases Black Widow on the Helicarrier, they did some mo-cap footage with an athlete in the mo-cap suit, and it looked too human for Joss Whedon. The end result looked too much like a real sprinter running, so they had to scrap it. Whedon wanted something more flexible and also more "out of control." For some action, they tried different actors in the mo-cap suit. And some cases, it's keyframe animation.

3. The Hulk has no penis.
They modeled every part of the Hulk, except for one. "When the maquette came in, it's just a Barbie doll," said Jason Smith.

4. They studied the corners of Mark Ruffalo's eyeballs
They captured Ruffalo "right down to the pore level," says White. "We did a cast of his teeth." They shot the corners of his eyeballs, so they could spread his eyes. They took a ton of images of the inside of his gums, and the space between his fingers. They studied his fingerprints. They captured every aspect of Ruffalo's stubble, and even every little ingrown hair. And every mole. They recorded the inside of Ruffalo's armpits. "The Hulk has a couple of scars that are straight from the source," says Jason Smith.

They also did a ton of photoreference. According to Smith, they had days when they were just focusing on the Hulk's eyebrows, and a week where "everybody was just doing Google image searches on teeth — and don't do that." They were wondering, "What color would Hulk's tongue be: red or green?" And what are the Hulk's fingernails like?

5. A makeup artist added cheek and brow attachments to Mark Ruffalo
Before they captured Ruffalo's performance as the Hulk, they had a makeup artist put attachments onto Ruffalo's cheeks and brow to make them more Hulk-shaped. And then they added what they called a "digital prosthetic" to enhance those features.

6. They chose not to make the Hulk look super buff, on purpose.
Previous movie versions of the Hulk looked really cut, with really sharply defined muscles. Like, the Edward Norton Hulk was "always kind of flexed" whenever he appeared, says Chu. But for this version of the Hulk, Whedon wanted more of a "wrestler physique," says Smith. So when they were modeling his body, they made him softer around the shoulders and stomach — so when the Hulk really goes berserk, he's got someplace to go, physically. His veins can pop out and his muscles can flex more when he's jumping around smashing aliens, than the rest of the time.

7. Hulk's motions were partly based on apes.
According to Chu, they studied simian motions for the Hulk — and when Mark Ruffalo came to the studio to experiment with different motions in the mo-cap suit, he "started to tend to go towards more apish motions, giving him that animalistic quality that gives you a feeling that he's not quite in control."

8. They debated how high the Hulk can jump
In the comics, the Hulk can jump for miles — but the makers of The Avengers wanted to keep him realistic and create a feeling that he had real mass, says Chu. So they decided he can jump to the tops of buildings, but "miles and miles away, probably not."

9. The shot where he turns into the Hulk and punches the Leviathan had to be redone.
The first time Bruce Banner becomes the Hulk, on the Helicarrier, he's being attacked and it's sort of involuntary. But the second time, he decides to become the Hulk, and walks up and punches the Leviathan, and it's a huge hero shot. When they first did that sequence, the transformation was really fast — it was basically over in a second or two. Whedon decided to go back and redo it so the change happened slower, and you could watch Mark Ruffalo turn into the Hulk, and see his clothes explode off him. They couldn't use the real Mark Ruffalo for that sequence at all — so the hardest part was the first frame, trying to get the digital Mark to look like the real Mark.

Iron Man

1. They totally changed how Iron Man flies
Joss Whedon "wanted to take off the training wheels" that Iron Man had in his first two movies, says Marc Chu, who had worked on both Iron Man films. In other words, Iron Man needed to be able to fly without using the thrusters in his hands and feet this time around. They added a "backpack thruster," and that enabled Iron Man "to make some comic book poses" instead of using his limbs to hover.

2. Robert Downey Jr. basically never wears the full Iron Man suit any more.
That suit is really, really uncomfortable and pinchy, says Chu. And whenever you see Iron Man in his armor, that's a CG rendering of Iron Man, or a stunt man named Clay. After the first Iron Man movie, says Chu, Downey Jr. saw what they could do with CG versions of the suit. "He knew he did not have to wear as much of the suit, and that would make him a lot more comfortable." There's a partial version called the "football suit" that he wears in a couple scenes, like at the end when he's laying on the ground.

3. They worked really hard to keep the Mark 7 armor from feeling like a "magical tortoise shell."
When all the pieces fly onto him, "the volume of pieces" had to feel real and not like they were coming from nowhere, says Chu.

The Other Avengers

1. When Hulk punches Thor, that was one of the hardest shots to get right.
That sequence took from the first day of the process until the very end, says White. It's one long continuous shot of the two of them working together, ending with Hulk punching Thor. For the actual punch, they put Chris Hemsworth in front of a bluescreen standing on top of a real section of a downed Leviathan that they built. And they attached Hemsworth to a cable pull that they retimed, so he could look like he was knocked sidewise. They cut a few frames out of the footage, so it looks like Hulk's punch has "instant impact," says White.

2. Every single Avenger had a digital copy.
Check out the Hulk video up top — they were able to substitute a CG version of Black Widow in some scenes where she's involved in a mostly CG action scene. Black Widow was the hardest to do, because a beautiful person is harder to model than an ugly person — her eyelashes had to be perfect, or it wouldn't look like the real Scarlett Johansson. Typically, you only get a limited amount of time to capture images of each actor, but in this movie, they got tons of high-quality scans of every actor, so they could create really high-quality digital versions. They captured every possible facial expression from these actors, using the Nova camera system.

3. Cap is digital in that big "hero shot".
The big scene where all the Avengers are fighting and the camera pans through, showing each of them in turn fighting different aliens, required some last-minute tweaks. Whedon decided that instead of fighting alone, Captain America should be helping Iron Man — so they cut Captain America out of his original location and put a digital Cap next to Tony. (They could also make Cap jump higher and spin-kick better than the real stunt guy.) Likewise, in the shot at left from the trailer, where they're all in a circle facing outwards, they cut Hawkeye and put him someplace else. (It was tough to get the Hulk in a shot with everyone else, because he's a big guy.)

4. When Thor uses Mjolnir to create a storm cloud, that's stock footage.
They were going to create a huge simulation of a storm over Thor's head, but in the end they just wound up buying a stock clip of storm clouds forming in a circle. It's a super-brief shot, so it wasn't worth creating CG clouds for.

Virtual New York

1. They only had three days to film in the real New York.
They originally thought they'd have a few weeks or a month of filming in Manhattan. But in the end, they only had three days, says Jason Smith. They got a few shots of all the Avengers standing on the real Park Avenue viaduct, that they intercut with the shots of the 300-foot viaduct replica that they built in New Mexico. It's really difficult to film in New York — you can't get clearance for a helicopter below 500 feet, you can't close the viaduct for days, and so on. New Yorkers keep saying that it's amazing that they were able to film so much in New York, not realizing it's mostly digital shots. (Plus Cleveland.)

2. They basically made their own Google street view of Manhattan, so they could model it.
The VFX crew had a team of still photographers, who went around taking "a massive collection of images," says Jeff White. In the video at left, the little chrome spheres raining down "represent all the photography we shot in New York City." Each sphere represents 72 high-res images that they shot, in all direction, so they could be sure of capturing every surface. They projected those images onto the buildings they created virtually, so they could render their camera moving through there.

But when you have a moving image, things like the reflections in windows need to move too — so they added their own actual office windows, in San Francisco, to the office buildings. If you look carefully, you might be able to see inside the ILM offices during the big battle scene. Some buildings, they rendered from scratch, like the Chrysler Building. They also drove around in a car with an Ultima arm rig, filming the streets for reference. So they could see what a particular building looks like at any particular time of day.

3. They spent a lot of time trying to keep New York's geography consistent.
So when someone is running or flying through the city, they tried to make this person's path logical — if they turned onto sixth avenue, they couldn't suddenly be in Alphabet City. After a jumbo jet crashes into a building, they made sure that building had damage when we saw it later.

4. They added some in-jokes to the storefronts in the city.
Like, of course, there's a shawarma restaurant in the background during the battle. There's also a store that sells something wacky like "books and sandwiches." And there's a law firm called Kirby & Lee, Attorneys at Law. (Note that it's Kirby and Lee, not the other way around.)

The Aliens and Loki

1. The Chitauri were originally way too glam. Like, Vegas glam.
Their armor was original a lot more golden. In the early designs, they looked really cool, with the gold armor looking menacing against their pale skin. But once you see it rendered in CG, it looked way, way too Las Vegas. "It started to look decorative," says Smith. So they pushed it more in the direction of looking bronze rather than gold. Ditto with the Leviathan, which the designers called "Jumbo" internally— he was originally a lot more blinged-out, and this made him look a lot more fake. So they ended up dirtying up the aliens a lot more, to take the super-bright gold look off them.

2. They were fishy.
The Marvel art department did some concept art where the Chitauri invaders have translucent skin that looks "almost fishy," says Jason Smith. So the ILM crew looked at lots of fish, especially "those angler fish that live at the bottom of the ocean," and manta rays. Also, the Chitauri's armor looked mechanical and "bolted into them" in the concept art, so they went with armor that looked kind of uncomfortable, and also weathered.

3. When the Hulk is smashing Loki up and down, they inserted Tom Hiddleston's real agonized face.
This took a lot of shooting of Hiddleston looking pained, so they could paste it into the digital Loki. "We're not inventing how he looks when he's in pain," says Chu. "I had to get behind [Tom Hiddleston] and shake him violently," so they could capture his real expressions. (He mimes violent shaking.) "I did it so long, he started laughing. So we didn't use that portion." And for one brief shot where Loki is just hanging upside down, they still had to cut Loki's face from somewhere else and stick it in, flipped the opposite direction.

"When we first heard about the thing where Hulk slams Loki up and down," says Smith, "it's like that's a bold thing to do. It's very cartoony. It's almost Hanna-Barbera." But when you watch it in the theater, it totally works, and it's the way we've always wanted to see the Hulk. And that's all down to Joss Whedon getting why these characters work, says Smith. In that shot, Whedon really wanted the Hulk's face to be totally deadpan, rather than making a lot of grimaces or weird facial expressions — and that's a huge part of why it works. "Because Hulk is just like, 'Yeah, I'm going to smash you into the ground, and it's not a big deal to me.'"

The Helicarrier

1. We learned how the Helicarrier turns invisible.
Maybe you already got this from watching the movie, but they decided basically the Helicarrier has a ton of cameras on top, photographing the actual sky, and then projecting that image onto LED panels on the bottom. Marvel wanted the Helicarrier to feel like something that could actually exist — but at the same time, it's a levitating aircraft carrier that comes out of the water and turns invisible, says Smith. And has ports that water come out of.